1. Field of the Invention
The present invention relates to a process for the preparation of diamond. More particularly, the present invention relates to a process for the preparation of diamond capable of preparing diamond in a film form applicable to film bodies for diaphragms for speakers, heat conductive plates, cutting tools, semiconductors and so on and capable of applying pulverized film bodies to abrasive, cutting tools and so on.
2. Description of Related Art
Diamond film can be utilized for various uses due to its superior characteristics such as hardness, heat resistance, rigidity, etc. Hence, research and development have been made for commercializing the diamond film in various fields.
Generally, the diamond film may be employed in a state in which it is formed on a substrate or it may be employed as a free-standing diamond film without the substrate. For example, a proposal has been put forward for using the diamond film as diaphragms for speakers and so on by taking advantage of its high rigidity. In this case, removal of the substrate is required from the diamond film formed on the substrate. However, there are many instances in which the diamond film is employed in such a state that it is formed on the substrate, so that demands have heretofore been made to improve adhesion of the diamond film to the substrate. Little research has been made so far on technology of readily separating the diamond film from the substrate on which the diamond film is formed or technology of readily removing the substrate from the diamond film formed thereon, and only some proposals have been put forward so far as will be described hereinafter.
For instance, Japanese Patent Publication (kokoku) No. 57,399/1988 discloses technology of separating a diamond film from a substrate with a small thermal conductivity, on whose surface the diamond film is directly formed, by heating the diamond film with heating light and taking advantage of difference in thermal expansion between the diamond film and the substrate.
Further, Japanese Patent Laid-open Publication (kokai) No. 143,498/1984 discloses technology which involves forming a diamond film directly on the surface of metallic substrate and treating the metallic substrate with NaOH or mixed acids consisting of hydrochloric acid, hydrofluoric acid and nitric acid, thereby dissolving the metallic substrate and removing it.
However, the technology as disclosed in Japanese Patent Publication (kokoku) No. 57,399/1988 cannot always achieve ready and uniform separation of the diamond film from the substrate because it takes advantage of the difference in thermal expansion between the diamond film and the substrate. Further, it may present the problem that the separation of the diamond film from the substrate causes distortion. In other words, as the diamond film is formed at temperatures as high as approximately 1,000.degree. C. and then cooled to approximately room temperature, the diamond film may be delaminated partially during the course of cooling the diamond film to approximately room temperature. Another problem is that selection of the substrate is restricted to certain materials because a large difference in thermal expansion is required between the diamond film and the substrate.
Further, the technology as disclosed in Japanese Patent Publication (kokoku) No. 143,498/1984 requires special chemicals for dissolving the metallic substrate, so that it suffers from the disadvantage that it is not efficient due to requirements for after-treatment such as washing, etc. Further, this technology presents the same problem that separation of the diamond film may cause distortion prior to treatment with the chemicals, as the aforesaid technology does.
On the other hand, Japanese Patent Laid-open Publication (kokai) Nos. 161,897/1986 and 161,898/1986 propose technology of using the diamond film in a multi-layer state in which it is integrally formed on a substrate such as alumina, Si.sub.3 N.sub.4, etc., without separation or removal of the diamond film from the substrate.
However, this technology cannot take advantage of or demonstrate the excellent characteristics inherent in the diamond film, so that it suffers from the disadvantage that such diaphragms for speakers as employ the diamond film in the multi-layer state are poorer in performance than those employing a free-standing diamond film.
In addition to the diamond film, diamond powders are suitable for use is abrasives and sintered bodies of the diamond powders are employed for cutting tools and so on.
Heretofore, the diamond powders have been prepared by synthesizing diamond under high pressure, pulverizing it into the particle sizes required, and then classifying it.
This process, however, has the drawbacks that an apparatus for synthesizing the diamond is too complicated and the resulting diamond powder contains impurities.
Recently, the synthesis of diamond by means of low-pressure vapor phase method using plasma has been established, thereby making it possible to prepare the diamond films of high quality.
Further, many attempts have been made to prepare diamond powders of high quality by taking advantage of the low-pressure vapor phase method.
For instance, Japanese Patent Laid-open Publication (kokai) No. 36,112/1986 proposes a process which involves scratching a surface of the substrate on which diamond is formed upon synthesis from vapor phase, arranging for the surface of the substrate to have a given degree of surface irregularity so as to form plural sites from which the diamond starts depositing, forming diamond particles on the substrate, and then separating the diamond particles from the substrate.
In this process, however, it is extremely hard to control the sites from which the diamond starts depositing. Further, adhesion of the diamond particles to the substrate is so strong that the substrate is required, for instance, to be dissolved with aqua regia or the like in order to provide the diamond particles only.
Further, Japanese Patent Laid-open Publication (kokai) No. 155,295/1986 proposes a method which involves scattering finely pulverized powders of at least one member of metallic silicon, an alloy thereof or a compound thereof to be employed as the nuclei for depositing the diamond on the surface of the substrate and using them for the CVD diamond synthesis method. This process can produce fine diamond particles on the surface-treated substrate with high productivity and at arbitrary density.
This process has the object to provide diamond film with good adhesion to the substrate, so that it presents the problem that the difficulty resides in separating the diamond from the substrate because the resulting diamond is in direct contact with the substrate.
On the other hand, a process for preparing fine diamond powders is proposed in Japanese Patent Laid-open Publication (kokai) No. 156,009/1988, which involves introducing a nucleus for synthesizing diamond into a plasma without forming the diamond directly on the substrate.
However, this process suffers from the difficulties that, since a large number of nuclei for forming the diamond are formed, only fine particles having particle sizes ranging from 0.3 micron to 0.5 microns can be formed. This use is restricted, and control over the particle size of the resulting diamond is hard.
Furthermore, Japanese Patent Laid-open Publication (kokai) No. 231,494/1985 proposes a process for preparing superfine diamond powders characterized by suspending inorganic fine particles, which are not caused to melt in plasma, in a reactor, and Japanese Patent Laid-open Publication (kokai) No. 137,311/1984 proposes a process for synthesizing diamond particles while vibrating and fluidizing powders of the metallic fine particles functioning as nuclei, such as metal fine particles.
This process, however, requires control over both of the state in which the powders functioning as nuclei flow and the reactive state, for example, in which plasma is generated. It is difficult to control both of the fluidizing conditions and the reactive conditions at the same time. Further, this process cannot produce diamond particles of high purity consisting of diamond alone, because the diamond is formed on the nuclei.
A further process for preparing diamond particles is proposed in Japanese Patent Laid-open Publication (kokai) No. 111,707/1989, which involves synthesizing the diamond particles on surfaces of fibrous metal or fibers such as glass fibers, carbon fibers or the like and removing, by dissolving, the fibers.
This process, however, has the drawbacks that it requires a large amount of fibers and the fibers become too bulky, so that the production efficiency of diamond particles is poor from the industrial point of view.
In other words, conventional processes are hard in controlling the synthesis conditions for the preparation of diamond particles or in controlling particle sizes for providing the diamond particles with given particle sizes. Further, they are very difficult to provide the diamond powders of high purity and of high strength.